The Global Positioning Satellites (GPS) system provides coordinates for navigation. GPS uses 24 to 27 half geo-synchronous satellites for triangulation positioning in three dimensions. GPS has many deficiencies in global coverage. Current GPS signals cannot be used indoors since GPS receivers need line-of-sight signal reception, and the signal level is too weak to penetrate building structure. It is also difficult to acquire the low elevation angle GPS satellites in city areas due to the blockage by high rise buildings. GPS requires large and expensive national resources to continuously manufacture, launch and maintain. In addition, the cost of GPS receivers is relatively high due to the complexity of the signal structure, the need to compensate interference, and ionosphere delays with relatively weak signals.
Prior to GPS, a Long Range Positioning (LORAN) system was used for navigation. The LORAN system uses hyperbolic positioning theory rather than spherical triangulation theory of GPS. The hyperbolic positioning uses time-difference-of-arrival (TDOA), while the spherical theory uses time-of-arrival (TOA). Both hyperbolic positioning theory and spherical triangulation theory must have the base stations perfectly synchronized in time for high precision. In LORAN systems, one primary high frequency (HF) radio station and two slave stations transmit pulsed signals with known periods and different frequencies. Legacy geometric theory, the constant difference line between two points is a hyperbolic curve. A pair of hyperbolic curve intersection points determines the position of an object. The position of the object is determined by the difference of distance (TDOA) from two pairs of stations. Modern LORAN systems (enhanced), can take advantage of GPS, or use synchronized atomic clocks for all stations to avoid the master slave station relationships.
An OMEGA system has also been used for long range locating using VLF (very low frequency) signals. The frequency band is in the range of 11 to 15 KHz, which has a very long wavelength and propagates for long distances. Different from LORAN, OMEGA uses phase difference of two continuous waveform signals to determine position. However, position accuracy is not precise (4 nautical miles) due to several reasons: The phase ambiguity of many cycles for long distances; the multiple paths of the waveform from ionosphere; the stratosphere, and ocean or ground reflected signals mixed with direct line of sight signals; and, clock synchronization errors of the base stations that are far apart.
A recent historical need of object locating system is triggered by the E-911 effort. It was mandated by the Federal Communications Commission (FCC) that the cell phone industry provide rough position information to law enforcement agencies for 911 calls. Due to the infrastructure of cell phone relay towers, the locating systems are more conveniently based on existing cell tower base stations. Base stations can be synchronized in time by the cell phone company via network or GPS time. The technologies used by GSM (Global System for Mobile communications) cell phones are TOA, AOA (Angle-Of-Arrival), E-OTD (Enhanced-Observed-Time-Difference), and A-GPS. The technologies used by CDMA (Code Division Multiple Access) cell phones are A-FLT (Advanced-Forward-Link-Triangulation). The technology used by TDMA (Time Division Multiple Access) cell phone is the A-GPS. A-GPS has accuracy of around 5 meters with networked assisted timing for reduced acquisition time. The AFLT and E-OTD have accuracy of around 100 meters. The TDOA and AOA have accuracy of about 150 meters. Other than GPS, the cell phone positioning accuracy is only good for E-911 purposes, and not suitable for indoor locating applications.
Modern indoor location systems use WiFi (Wideband Fidelity), or other LAN (Local Area Network) signals for triangulation based on multi-stations Radio Signal Strength (RSS), TOA, TDOA or a hybrid of the above techniques. The problem for such indoor signal propagation is the uneven attenuation and reflection caused by walls, room partitions and metal furniture. Their accuracy is degraded even at short indoor distances at high frequency (2.4 GHz). In addition, it needs the existence of well established WiFi network architecture.
Other locating systems attempt use of commercial broadcasting AM radio signals phase correlation for navigation. Such approaches relied on commercial AM frequencies, ranging from 520 KHz to 1710 KHz (approximately 200 meters to 600 meters in wavelength), which is suitable for indoor use, but the signals can suffer significant distortions around large conductors inside structures.
Since AM signals can be attenuated easily by blockage and building, FM signals vary in phase, another approach has been to use the constant envelop signal strength of FM signals rather than the phase, for location base service (LBS). This approach (called SPOT watch) uses a miniature FM signal receiver installed inside a watch size display device for LBS services. The radio signal strength (RSS) ranking of a number of FM stations is used to determine the neighborhood of a city for LBS. By using the statistical ranking of 5 to 11 stations with simulated signal strength, a rough position within 5 miles of township can be located. However, the position accuracy of 5 miles is imprecise, though it serves the purpose of LBS.